It'll reduce back pressure, lean the mix a touch and righteously piss off the neighbors, pedestrians, and the random law enforcement officer while providing only a minor gain in performance.
Many do it and enjoy the effect, though a more reasonable alteration would be a reduction of the internal baffle and/or drilling holes but leaving the end cap on.
Personally... no offense intended really, but I think it kinda halfassed and a touch ghetto insofar as a "performance" mod goes. Aftermarket expansion chambers can be really inexpensive and while reducing back pressure helps - it's really the pressure wave reflection that results in real performance with a two stroke.
Usually I'm content with the philosophy "to each their own" and if ya digit - so be it... But what with the legal shaky ground in regards to these MBs I'm just not gonna be all that enthusiastic about something that isn't going to do much more than anger everyone other than the rider himself. Honestly, much as I dearly love these bikes - if I had a neighbor who pulled the endcap... I'd be tempted to mace the SOB lol